Low pressure on two out of four zones

I have a low pressure issue on two out of my four zones. The two zones that work fine are in my front yard which are zones 1 and 2. The two that have low pressure are in the back yard, zones 3 and 4.

The previous home owners have a tap for a water pressure gauge in line so I can see the water pressure when each zone comes on. When zones 1 and 2 come on, the pressure is around 70 psi, but when zones 3 and 4 come on, it drops to around 30 psi. I had a sprinkler repair guy come out and he said that he thinks the issue is a leak or root rapped around my main line, which is confusing to me because if that were the case, I would think I would see low pressure to all zones if that were the case. Any suggestions? Would it be worth replacing the valves to zones 3 and 4? So far I have been unable to find any leaks.

You can cap off heads to help find leaks, or to rule out leaks. Spray heads often have throttle screws that can close off flow. Rotor heads are more involved, sometimes requiring they be unthreaded and a plug or cap being temporarily put in their place. If you have the skill to change rotor nozzles, and have the spares that you should have for your inventory, you can take some of the nozzles that are sizes you know you'll never need, and fill the inside cavities with silicone caulk, turning them into "nozzle plugs" that will aid your diagnosis.

Referring to the pressure gauge readings, a supply pressure below normal points at a leak. When there is root constriction choking a zone pipe, the flow is reduced, and the supply pressure should become higher than normal.